As many parents have discovered, one of quickest ways to stop babies from crying is to pick them up and carry them around. Holding the infant in your lap doesn't have the same sedating effect. Indeed, some babies have a tendency to start crying as soon as you sit down again. Are they playing power games? Or does movement bestow a calming effect?
Researchers at the RIKEN Brain Science Institute in Japan think the answer is rooted in evolutionary biology. Simply put, in times of danger, it is easier to flee with a child who isn't kicking and screaming. So the simple act of being carried translates into a signal for the child to be still.
The Japanese researchers say a similar behaviourial pattern can be found in many other mammals, ranging from monkeys to mice. To advance their theory, they conducted a series of experiments in human babies and mouse pups.
In one experiment, the researchers attached electrodes to an infant to record his heart rate. "The lower the heart rate, the more relaxed the child," the lead researcher, Gianluca Esposito, explained in a video released with the study, published on Thursday in the journal Current Biology.
The mother was asked to alternate between holding the child while sitting down and carrying the child while walking.
"When the mother is sitting, the baby tends to be crying and actively moving. As soon as the mother stands up and moves, the heart rate decreases sharply," Esposito said. The child appears to be relaxed and his crying comes to an abrupt halt.
A similar calming effect was seen in studies of laboratory mice. Of course, a mouse can't carry the pup in its limbs. Instead, the mother will use its mouth to grasp the pup's skin at the back of the neck. When the pup is picked up in this fashion, it instantly becomes calm and is easier for the mother to carry.
"The infant response reduces maternal burden of carrying, and is beneficial for both the mother and the infant," the senior researcher, Kumi Kuroda, said in a statement.
As part of their study, the researchers also investigated what happens when a mouse pup can't sense it has been picked up. In some of the pups, they injected a chemical that temporarily deadened the nerves at the back of the neck. When these pups were grasped by their mothers, they continued to struggle, making it much harder for them to be carried. If a predator was nearby, such a response would "hinder maternal rescue [efforts] significantly," Kuroda noted in an e-mail.
The researchers hope that their findings will lead to a better understanding of infant behaviour and reduce some of the frustrations of parenthood.
"This study shows that infant-carrying is a good soothing measure for crying caused by mild irritations such a vaccination or a frightening noise," Kuroda said. "On the other hand, the robust calming effect is limited to the actual period of maternal carrying. So the baby may resume crying once he is put back in bed."
Some psychologists have previously speculated that the resumption of crying is a deliberate attempt on the part of the child to control the parents. But Kuroda and her colleagues disagree with this interpretation of the child's behaviour. "This phenomenon should simply be interpreted as a natural consequence of the infant sensory-motor systems," she said. "Such proper understanding of infants would reduce frustrations of parents and be beneficial, because frustration of unsoothable crying is a major risk factor for child abuse."